The preparation of the surface of steel or other iron-containing workpiece to receive a coat of ceramic (often also called enamel-coat or porcelain enamel) has presented a number of problems to the industry if a tightly adherent, defect-free ceramic coat is to be obtained. Preparation of iron-containing workpiece for enameling presently involves a long and fairly complex series of operations performed in both hot and cold solutions which require careful control of temperature, pH, and concentration.
In one practice, two ceramic coats are sequentially applied, the first being called a ground coat and the second, a cover coat. For each coat, it is necessary to use a special frit composition. Each frit composition is milled, dried, and after application to the metallic workpiece, each coat must in turn be fired.
In order to eliminate the ground coat application and all that it entails, so-called direct-on or a single application of an enamel or ceramic coat has been proposed. In this case, the application is limited to special and expensive grades of steel, such as zero carbon steel which may contain up to about 0.003 percent carbon. Further, it is necessary to etch the surface of such workpiece, as with sulfuric acid, as a preliminary step which ordinarily is more extensive than for a ground coat and cover coat application. For example, the acid etch may remove as much as 3.5 grams per square foot of zero carbon steel which is relatively expensive.
Not only does a workpiece previously designed for direct-on enameling require a more severe etch, but it also generally requires a heavier deposit of nickel to obtain adherence in the absence of the use of highly colored adherence oxides, such as cobalt oxides, found in ground coats. Since the higher carbon contents of steel like cold rolled steel result in unacceptable enamel defects if ground coats are omitted, these grades of steel have not been successfully used for direct-on ceramic or enamel coating. If a direct-on ceramic coat is attempted on a carbon-rich steel substrate, such as cold rolled steel, the resulting ceramic coat not only has poor adherence but is subject to fish scaling and reboiling defects. Bubbles, pits or specks almost inevitably develop in the surface of a single ceramic coat applied to such a substrate, marring its uniformity and texture. The ceramic coat is also more prone to chipping.
The art has previously attempted to adapt carbon-rich iron alloys for a direct-on or one-fire ceramic coat. In U.S. Pat. No. 2,819,207 to Shepard, for example, the disclosed important aspect of the process of that patent is said to be the provision of a finely divided coat of cobalt or nickel on a cleaned surface by means of an electrodeposition procedure. U.S. Pat. No. 3,078,180 to Zander et al. discloses a pickling and etching technique which includes closely controlled treatments of an enameling stock in solutions of ferric sulfate and sulfuric acid, so that scale and rust are effectively removed and fine grained, uniformly etched metal surface is said to be obtained.